here is my opinion: and i cant test this right this moment to be sure. but when u attack with either of the spin dash moves, the only way to cancel it is to use your 2nd jump. so that means-
you will spin, and if your on the ground, you will jump off the ground(still in the spin), and then after your spinning in the air you can cancel the spin with the 2nd jump. now you cant use a smash unless your on the ground, and trust me, if you go back to the ground after hitting your opponent with the spin dash, it would be kinda hard to land and then get under them for a Usmash. Somtimes the oponnent lands on the ground first too... so it would have to be jjust right, and u would have to be at the right spot, or get to it hella fast.
I mean, im sure in certain situations where everythign is perfect, a Usmash would go great after a spin dash attack... but berely ever i'd assume.
in my opinion its a way better idea to try and land an arial on them after a spin dash hit, its just more reliable in general.
theres my two cents